Jon Elson wrote:

oparr wrote:
This is uncharted territory for me. Done similar stuff with wood using Brad
point and Forstner bits but never with metal. Basically, I need a
counterbore (CB) diameter of about 3/4" to a depth of about 3/8" in 1" thick
6061 aluminum. This is to create a recess for cap screw head and washer
along with room to use a hex socket on the cap screw.

Sourcing the correct CB and pilot seems easy enough except that MMC
recommends HSS ($$) for ferrous and carbide tipped ($$$$) CB for non-ferrous
metals. Surely, HSS should work for aluminum or am I missing something?
Also, what would be the best drill speed to use in a case like this? I
normally use wax lubricant for countersinking metal, is this good enough for
counterboring? Finally, is it advisable to buy say a 1/2" aluminum blank and
practice first or am I making a mountain out of a mole hill?


HSS will work fine, if your drill press will turn slow enough. For a
3/4" counterbore, you could use a 3/4" end mill, but maybe with a drill
press a real counterbore tool is safer. I assume you have a Morse
taper to Jacobs taper arbor attaching the drill chuck to the spindle
of the drill press? These can come off suddenly when there are side
forces or chatter on the tool. You probably want to keep the
counterbore well under 1000 RPM for best results.



If you are going to go with the end mill one way to make things a little
safer is to:

1. Drill the hole for the "bolt".

2. Drill to almost 3/8 inch with a 3/4 drill.

3. Finish up with a 3/4 end mill to make the bottom flat.
Lower the end mill into the hole made by the drill before
starting the drill press. A foot switch on the drill press is a
great help and lets you have one hand for the quill feed and
on hand for the work.

Ray Spinhirne




Jon